A program is underway which deals with capsid morphogenesis in T4 and with the mechanism of genetic recombination in T4. The T4 program is concerned primarily with genetic control of capsid form and has isolated mutant strains characterized by short (petit) and long (giant) capsids. The altered protein is the principal structural protein of the capsid whose amino acid sequence is encoded in gene 23. Recently other mutants have been isolated which produce variation in capsid diameter. Identifying (by genetic mapping) the protein altered here is one main objective of current work. Isolation and mapping of additional categories of mutant affecting capsid form will be continued. The geometric structures of aberrant capsids will be compared to the normal T4 capsid with the aim of understanding capsid structure and the assembly process at the level of the structural and enzymatic proteins involved. The T5 experiments are initially aimed at construction of the total genetic map of that phage. Thereafter we will investigate the recombinational influence of the single-strand interruptions which are located at specific genetic locations in the infectious viral particle. Finally, attempts will be made to elucidate the manner in which the genome is packed into the T5 capsid.